The present invention relates to a power converter for converting AC power to DC power or converting the frequency of AC power to a different frequency and, more particularly, relates to a power converter having a function of reducing the reactive power of an AC power supply.
FIG. 1 shows a circuit configuration of a power converter which involves a problem to be solved by the present invention. An AC power supply 1 is connected to the primary of each of first and second power transformers 2 and 3. Transformers 2 and 3 are isolated from each other and constitute two separate power sources. The secondary of transformer 2 is connected to a first thyristor bridge 4 being formed of GTO (gate turn-off) or self-extinction type thyristors. The secondary of transformer 3 is connected to a second thyristor bridge 5 being formed of general thyristors. Reference numeral 6 denotes a filtering power reactor, 7 denotes a load, and 8 denotes a surge absorbing circuit consisting of resistors and capacitors. In this circuit configuration, the triggering for first thyristor bridge 4 is controlled in a phase-advanced mode, while the triggering for second thyristor bridge 5 is controlled in a phase-delayed mode. Then, phase-advanced reactive power and phase-delayed reactive power generated by the respective thyristor bridges are cancelled, so that the reactive power of AC power supply 1 is reduced. Thus, the power converter of FIG. 1 can operate with a unity power factor for the fundamental wave.
In the configuration of FIG. 1, when the thyristor bridge is controlled in a general phase-delayed mode, a commutation is performed, without requiring a self-extinction function by the thyristors, by simply triggering the thyristor to be supplied with a current of the next triggering sequence. In this case, a current flow is shifted or commutated from the thyristor in a current feeding state to the thyristor to be triggered next, and the current feeding phase of the power supply is changed. Such a commutation is called a natural commutation. According to a natural commutation, a current is sequentially switched in accordance with the difference in the phase voltages of power sources 2 and 3, and substantially no surge voltage is generated.
In contrast to the above, when the thyristor bridge is controlled in a phase-advanced mode, phase-advance/phase-delay relations of the phase voltages from power sources 2 and 3 are reversed. In this case, mere triggering for thyristors to be triggered next does not ensure a complete commutation. Thus, thyristors in a current feeding state must be positively turned off by a suitable means. In a GTO thyristor, this turning off operation is performed within a quite short period, so that the current flowing through the transformers is instantaneously cut off. From this, a surge voltage, caused by electromagnetic energy stored in the leakage inductance of transformers and other inductances of associated wirings, is generated. Surge absorbing circuit 8 is provided for suppressing such a surge voltage. Most of the electromagnetic energy is consumed as power loss in surge absorbing circuit 8. Although it depends on the value of the leakage inductance of transformers, this power loss amounts to several % to ten % of the handling power, and the power conversion efficiency is correspondingly degraded. This is the problem of the configuration of FIG. 1.
FIG. 2 shows a circuit configuration of another power converter whose power conversion efficiency is improved, but it still involves a problem to be solved. The circuit configuration of FIG. 2 is substantially identical to that disclosed in FIG. 4 of Japanese Patent Publication No. 51-22606. The surge absorbing circuit of FIG. 2 is formed of a rectifier 9, capacitor 10, filtering reactor 11, thyristor bridge 12, and transformer 13. A surge voltage generated is rectified by rectifier 9 and the rectified voltage is temporarily stored in capacitor 10. The energy stored in capacitor 10 is regenerated to power supply 1 via filtering reactor 11, thyristor bridge 12 and transformer 13 according to the inverting operation of thyristor bridge 12. In this circuit, although the power conversion efficiency is improved, a bulky device for surge absorption is required. The power converter thus becomes large in size and high in manufacturing cost. This is the problem of the configuration of FIG. 2.